N = 2 Gauge Theories on Systems of Fractional D 3 / D 7 Branes 1

We study a bound state of fractional D3/D7-branes in the ten-dimensional space IR × IR/Z2 using the boundary state formalism. We construct the boundary actions for this system and we show that higher order terms in the twisted fields are needed in order to satisfy the zero-force condition. We then find the classical background associated to the bound state and show that the gauge theory living on a probe fractional D3-brane correctly reproduces the perturbative behaviour of the corresponding N = 2 four-dimensional supersymmetric gauge theory. Work partially supported by the European Commission RTN programme HPRN-CT-200000131, by MURST.